This study addresses adaptation to the unique environment of parabolic flight. Experiments will take place in a NASA aircraft that flies parabolic trajectories which make transitions between 0 g and 1.8 g approximately every 25 sec. This unusual motion environment, with changes in g level not normally experienced, presents a remarkable challenge to the body's adaptive processes. Indeed, many first-time flyers experience disorientation and motion sickness. Nevertheless, adaptive processes prevail and on the second and subsequent flights the experience is much more pleasant, indicating that significant adaptation has occurred in a very short time and after a very brief exposure (approximately one hour of parabolic maneuvers per flight). We propose to study the adaptive processes that take place in this situation. A battery of tests of vestibular and oculomotor function will be performed during the two different gravity levels in flight. The tests span a range of neural processes from low-level reflexive through higher-level cognitive, and are aimed in particular at otolith information (since these vestibular organs sense linear accelerations, including gravity) and vision (since visual information can in many cases substitute for vestibular information about self-orientation). The tests are: 1) ocular counterrolling (OCR), a reflexive torsional movement of the eyes in relation to head tilt in a gravity field as sensed by the otolith organs; 2) translational VOR (TVOR), a compensatory eye movement made in response to head translations that also depends on processing of otolith signals; 3) pitch angular VOR, which involves convergence of otolith and canal signals; 4) vertical ocular alignment, which is affected by gravity level; 5) subjective visual vertical, based on multi-sensory integration of orientation cues; 6) roll vection, which addresses the interaction of visual and otolith cues. Responses from new and from experienced flyers will be compared, and the progression of responses over consecutive flights will be assessed as adaptation develops. The tests address two competing hypotheses of how adaptation to parabolic flight is achieved: 1) there is adaptation to each separate gravity level (context-specific); 2) adaptation is more generalized to the overall flight experience (implying a non-g-specific change in sensory weighting). The information to be gained from this study of adaptation may lead to better understanding of the range of possible adaptive mechanisms, and might help us to determine those mechanisms which act most rapidly and effectively (as they do in parabolic flight) as candidates for use in rehabilitation of vestibular patients.